1. Field of the Invention
The present invention relates to a packet communications system transmitting a packet at a predetermined transmission rate.
Especially, the present invention relates to a packet communications system using an HSDPA (High speed downlink packet access) scheme which is a packet communications technique for achieving a speedup of a peak transmission rate, a reduction of transmission delay, an increase in throughput and an improvement of radio resource usage efficiency.
2. Description of the Related Art
FIG. 1 shows a packet communications system using the HSDPA scheme. As shown in FIG. 1, the packet communications system is configured with a core network CN and a radio access network RAN. The radio access network RAN includes a plurality of radio network controllers RNCs and a plurality of base stations (Node Bs).
The radio access network RAN is connected to a plurality of mobile stations (UEs: User Equipments) via radio interfaces. As shown in FIG. 1, the radio access network RAN transmits an HS-DSCH (High speed-Downlink Shared Channel), which is a transport channel for downlink packet transmission, to a mobile station UE.
Referring to a flowchart shown in FIG. 2, an operation of changing a transmission rate of a downlink packet in the conventional packet communications system will be described.
As shown in FIG. 2, in step S601, an RLC (Radio Link Controller) connection is established over a DTCH (Dedicated Traffic Channel), which is a logical channel mapped to the HS-DSCH, between the radio access network RAN and the mobile station UE.
At the time when the RLC connection has been established in step S601, the transmission rate of the downlink packet (a transmission rate of the HS-DSCH or the DTCH) is “TX1” which is smaller than or equal to “384 kbps”, and a size of the downlink packet (RLC-PDU: Radio Link Connection-Packet Data Unit) to be transmitted via the RLC connection is “336 bits”.
In step S602, the radio access network RAN transmits the transmission rate of the downlink packet, when an amount of data of the downlink packet to be transmitted increases. For example, the transmission rate of the downlink packet is changed to “TX2” which is larger than “384 kbps”.
In step S603, a Re-establishment of the RLC connection between the radio access network RAN and the mobile station UE is generated, when the transmission rate of the downlink packet is increased.
Is step S604, the size of the RLC-PDU is changed to “656 bits”, after the Re-establishment of the RLC connection is generated.
The above change of the size of the RLC-PDU is performed in consideration with transmission efficiency.
However, there is a problem in that the Re-establishment of the RLC connection when the transmission rate of the downlink packet is changed, so that the packets stored in a transmission buffer are deleted and packet transmission throughput is reduced.